This invention relates to a method of forming bulk coatings of high transition temperature Nb.sub.3 Ge superconductors by the coreduction of NbCl.sub.5 and GeCl.sub.4 vapor in the presence of hydrogen, and more particularly to a method in which the requisite NbCl.sub.5 vapor is formed at a temperature of about 250 .degree. l to 260.degree. C.
The art discloses that the superconducting material with the highest known transition temperature is niobium germanide (Nb.sub.3 Ge) having an A-15 structure. A transition temperature of 22.5 K has been measured using small sections of an extremely thin film (&lt; 0.1 .mu.m) of Nb.sub.3 Ge deposited by sputtering under vacuum conditions. Bulk coatings are required, however, for practical applications, e.g., in superconducting power transmission, and such coatings may not be economically produced by sputtering techniques. The literature now teaches that such coatings may be produced by chemical vapor deposition in which NbCl.sub.5 and GeCl.sub.4 vapors are coreduced in the presence of hydrogen.
It is known that the NbCl.sub.5 vapor may be produced either by the vaporization of NbCl.sub.5 powder or by the quantitative chlorination of Nb metal at temperatures in excess of 800.degree. C. Both of these methods for producing NbCl.sub.5 have certain disadvantages. Thus, for example, vaporization from the powder requires both a uniform powder size and a very uniform powder feed. There is a tendency for the powder to agglomerate to some extent. The disadvantage of the prior art chlorination technique is the necessity of keeping the salt vapor at a relatively high temperature from production to reaction. Failure to do this can result in decomposition to NbCl.sub.3 which destroys the quantitative aspects of the process.